1. Field of the Invention
This invention relates to poly(butyleneterepthalate) molded articles and more particularly to mineral filled poly(butyleneterephthalate) molded articles having improved impact properties.
2. Description of the Prior Art
Poly(butyleneterephthalate) polyester resins have recently found utility in the manufacture of molded articles because of their excellent mechanical and physical properties. These thermoplastic polymers are suited for molded articles where impact strength, extensibility, toughness as well as excellent electrical properties are required.
The poly(butyleneterephthalate) resins are used primarily in molded applications to form a unitary article with the desired physical and mechanical properties.
In many instances, it is unnecessary to have the maximum strength imparted by a 100 percent poly(butyleneterephthalate) resin article and proposals have been made to reduce the strength of the poly(butyleneterephthalate) by adding fillers which also reduce the cost of the final article. The balance of reduced cost with reduced physical properties must be somewhat proportionate since small gains in cost reduction with large decreases in impact strength are undesirable.
Typically, a filler level of 10 to 50 percent by weight of the final molded article is acceptable from a cost standpoint, however, typical fillers such as talc, silica, asbestos and similar fillers demonstrate great reductions in impact strength at these filler levels. This degradation in impact and other physical properties is thought to be attributable to the formation of stress concentrations in the vicinity of the individual filler particles causing propagation of cracks in the molded article.
Further, because of the need for effective filler materials, generally, various mineral forms of inorganic compounds have been investigated. One mineral form of silica, novaculite, for example has been found to be not only a filler but also a reinforcer for poly(ethyleneterephthalate), U.S. Pat. No. 3,740,371. This novaculite filler has been used in poly(ethyleneterephthalate) having intrinsic viscosities of 0.28 to 0.97, the filling difficulties in the poly(ethyleneterephthalate) are due to the high melt viscosity of the polymer per se along with further combination with the filler.
Poly(ethyleneterephthalate) differs from poly(butyleneterephthalate) in that the molecular weights encountered with poly(butyleneterephthalate) are higher as measured by intrinsic viscoisities along with the attendant chemical differences. Thus, although a particular filler may be useful for one polymer it may not be effective for another, and hence fillers for poly(ethyleneterephthalate) do not necessarily perform the same in poly(butyleneterephthalate). Thus, in accordance with the present invention a mineral filled poly(butyleneterephthalate) is provided with improved impact strength over conventionally filled poly(butyleneterephthalate)s.